Reacting orbitals

The first interaction has favorable orbital phase overlap for a concerted (2ns + vizs) reaction. The interaction integral y, Eqs. 3—6, for a concerted process would have a maximum value if the two molecules approached each other so that the reacting orbitals could overlap in the most efficient manner. The best geometry would involve a face-to-face reaction of the two reactant species. The stereochemical consequence of such a reaction would be specific retention of substituent relative geometries. 

When the overlap of the reacting orbitals or product orbitals occurs on the opposite faces of the reacting molecules (called antarafacial way). 

Such cycloadditions involve the addition of a 2tt- electron system (alkene) to a 4ir- electron system (ylide) and have been predicted to occur in a concerted manner according to the Woodward-Hoffmann rules. The two most important factors involved in the cycloaddition reactions are (i) the energy and symmetry of the reacting orbitals and (ii) the thermodynamic stability of the cycloadduct. The reactivity of 1,3-dipolar systems has been successfully accounted for in terms of HOMO-LUMO interactions using frontier MO theory (71TL2717). This approach has been extended to explain the 1,3 reactivities of the nonclassical A,B-diheteropentalenes <77HC(30)317). 

The transition state for conversion of 2 to 3 is particularly reasonable because it combines some of the geometry of both the reactants and the products and therefore gives the best overlap of the reacting orbitals necessary for the formation of the 7r bond. This is shown more explicitly below.9... 

These reactions are 4n-electron concerted processes controlled by the symmetry of the reacting orbitals. The thermal reaction is most favorable with a... 

Intrinsic symmetry of reacting orbitals Third, in making bonding models, we noted that it is not always necessary to use all the symmetry elements of the molecule. We may be able to pick out one or more of a number of symmetry elements that will give the desired information. We shall find examples of this procedure in applications of the pericyclic theory. We also need another idea, already introduced in Section 10.4, namely that in some circumstances it is appropriate to use a symmetry element that is not strictly, but rather only approximately, a correct symmetry element of the molecule. The reason we can do this is that in pericyclic reactions we shall focus on those orbitals in the molecule that are actually involved in the bonding changes of interest, which we... 

The reacting orbital system needed to include all the bonds being formed or broken is made up in the reactant from a basis consisting of a p orbital on each of the four carbon atoms, and in the product of p orbitals on each of the two it bonded carbons and a hybrid orbital, approximately sp3, on each of the two carbons linked by the newly formed a bond. Figure 11.6 illustrates the basis functions and the molecular orbitals that arise from them. 

This reaction is a reverse [4 + 2] cycloaddition. The reacting orbitals have the correct symmetry for the reaction to take place by a favorable suprafacial process. 

The low Sn2 reactivity of 1°-alkyl bromide, 2,2-dimethyl-1-bromopropane (neopentyl bromide, 2.5), is explained by steric hindrance to the required 180° alignment of reacting orbitals. However, under Sn 1 conditions, neopentyl bromide (2.5) reacts at roughly the same rate as other 1°-alkyl halides such as ethyl bromide. Ionization of alkyl halides to carbocation in SnI is the rate-determining step. Although the product from ethyl bromide is ethanol as expected, neopentyl bromide (2.5) yields 2-methyl-2-butanol (2.6) instead of the expected 2,2-dimethyl-1-propanol (neopentyl alcohol) (2.7). This is because once formed the ethyl carbocation can only be transformed by a substitution or elimination process. In the case of the neopentyl carbocation, however, the initially formed l°-carbocation may be converted... 

For stereoelectronic reasons (overlap of reacting orbitals), the two reacting groups, H and X, must be either antiperiplanar or synperiplanar. During an electron transition there should be the least change in the position of the atoms involved (Frank-Condon effect). Strong bases with a large steric requirement will suppress the S 2 mode of reaction. 

Strong bonds require good overlap of the bonding orbitals. The formation of sigma bonds requires approximate colinearity of the reacting orbitals. The formation of pi bonds requires approximate coplanarity of the reacting orbitals. 

In the absence of a catalytic agent (such as light or chemicals), only the ground state HOMO and LUMO are the reacting orbitals. For the converse, excited state HOMO and LUMO may take part in orbital overlap. 

Now, look at the molecular orbitals for the prototype Diels-Alder transition state in order to identify the six reacting orbitals outlined above. 

This reaction is an example of a 1,3-shift that is suprafacial for both components and involves two 7i-systems, each with 3 electrons. The MOs of 1,5-hexadiene and the Cope-rearrangement transition state show the reacting orbitals. Table 4.3 gives the Woodward-Hoffmann rules for sigmatropic rearrangements between 7C-systems with I and / electrons. 

The Cl and C4 lobes of the HOMO of butadiene have opposite S3rmmetry (one lobe is and one is - ). As shown in transition state 4, the Cl and C4 carbons of butadiene are those where new carbon-carbon bonds are formed, so the symmetry of these orbitals is important. The orbitals of the diene must react with the molecular orbital of ethene in the LUMO, and the reactive LUMO has one + lobe and one lobe. The S3rmmetry of the reacting orbitals matches this is important to the reaction. If the symmetry of the orbitals does not match, the reaction does not occur by this concerted mechanism. 

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